GraphQL Playground is a full-featured GraphQL IDE (similar to GraphiQL) that provides an interactive development environment for GraphQL APIs. It includes a React-based frontend with query editing, schema exploration, documentation, subscriptions, and history. The repository is a monorepo (Lerna + Yarn workspaces) containing packages for the core React app (`graphql-playground-react`), an Electron desktop app (`graphql-playground-electron`), an HTML embedding page (`graphql-playground-html`), and server middleware for Express, Koa, Hapi, and Lambda. The architecture is a typical React + Redux + Redux-Saga + Immutable.js stack with CodeMirror for editors and Apollo Link for network execution. The project is now archived; future development has moved to GraphiQL. The codebase shows signs of age (outdated dependencies, legacy lifecycle methods, global variable assignments) and has had two documented XSS vulnerabilities. Despite this, some components like the schema fetcher, theming system, and middleware wrappers demonstrate solid engineering patterns.